A metal-oxide-semiconductor field-effect transistor (MOSFET), or simply a field-effect transistor (FET) or a transistor, is widely used in integrated circuits (ICs), including digital integrated circuits, radio frequency (RF) analog circuits, etc. The gate length of a transistor is reduced or scaled down to increase the packing density of transistors in an IC and to increase the speed performance thereof. However, transistors with aggressively miniaturized gate lengths suffer from undesirable short-channel effects, such as increased off-state leakage current.
One way to address suppressing short-channel effects is to employ a semiconducting channel with reduced thickness, referred to as an ultra-thin body transistor. Ultra-thin body transistors may employ ultra-thin channel materials. 2D materials, also referred to as few-layer materials, are crystalline materials of a few layers of atoms that are promising candidates for use for thin channel materials. One type of 2D material with high mobility is black phosphorus (BP). BP is a layered material; a monolayer of BP termed “phosphorene”. BP is a semiconductor with a direct band gap ranging from about 1.5 eV of a monolayer to 0.59 eV of a five-layer stack at the Γ point of the first Brillouin zone.
Existing transistors and methods of fabricating transistors with a BP, or other 2D materials, channel can have undesirable contact resistance which limits their performance. For example, some methods can provide radio frequency (RF) transistors using BP as a channel material can exhibit current gain, cutoff frequency (fT) and maximum oscillation frequency (fmax) that are lower than desired.